This is the "step", "skip" or "by" frame specified if you specify your frame range as:
1-10x2
This would do every 2nd frame between 1 -> 10 (the odd-numbered) frames, and in this case the QB_FRAME_STEP would be "2".
If you wanted to render every 5th frame between 1 and 100, you would say:
1-100x5
and QB_FRAME_STEP would be "5"